Alkoxide Research Articles

Hydrogen and carbon dioxide uptake on scalable and inexpensive microporous carbon foams

Hierarchically porous carbon foam (CF) and microporous nitrogen doped carbon foam (NCF) were synthesized at gram scale via the one-step thermal decomposition of metal alkoxides. The total hydrogen uptake of CF reached a maximum of 11.0 wt% at 77 K and 9.5 MPa (or 5.2 wt% excess at 3.9 MPa). This large uptake is attributed to large surface area (3452 m2/g) and pore volume (2.19 cm3/g). Even at room temperature, the total hydrogen uptake reached 2.50 wt% (or 0.94 wt% excess). Nitrogen doping resulted in lower hydrogen uptake at higher pressure, due to the lower surface area. Interestingly however, slightly improved hydrogen uptake was obtained in NCF at lower pressure compared to CF, and we attribute this to the narrower pore size. Meanwhile, the CO2 uptake of CF was 15.2 mmol/g at 273 K and 0.5 MPa. The CO2 uptake of NCF was slightly lower, but the CO2/N2 and CO2/H2 selectivity were higher. This was attributed to increased isosteric heat of adsorption between CO2 and the nitrogen-doped carbon surface. This work shows the potential of bulk-synthesized low-cost metal alkoxide-derived carbon foams to be used in gas storage and separation applications.

Cover Picture: Flow Synthesis of Single and Mixed Metal Oxides (Chem. Methods 8/2022)

The Front Cover shows a versatile flow synthesis strategy for continuous manufacturing of single- and mixed-metal oxide particles with a high degree of size monodispersity. The flow-focusing microreactor equipped with an online photo-crosslinking module enables facile production of a broad range of monodispersed metal oxide particles (ZnO, SnO2, CeO2, LaPrO3) using metal organic precursors beyond metal alkoxides. More information can be found in the Research Article by Zachary S. Campbell et al..

Recent Advances in Base-assisted Michael Addition Reactions

Abstract: The Michael addition is an important, highly efficient, and atom-economical method for the diastereoselective and enantioselective C-C bond formation. MA used in the synthesis of natural products and drugs is tandem sequenced reactions. An important tandem sequence of Michael and Aldol additions is the Robinson annulation. MA is a versatile reaction employed for efficient bond formation between electron-poor olefins as Michael acceptors with varied nucleophiles as Michael donors. Apart from being involved in C-C bond formation, MA is also employed in the formation of C-X bond (X=O, N, S, P etc.) giving rise oxa-, aza-, thia-, phospha-Michael addition products. In recent years many articles have been published on MA. The mechanism of MA gives an insight into the reaction is initiated by the base. The present review provides comprehensive information on recent advancements in base-assisted Michael addition reactions including varied organic and inorganic bases, such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,4-Diazabicyclo[2.2.2]octane (DABCO), 4-dimethylaminopyridine (DMAP), triethylamine (Et3N), tributylamine (Bu3N), N,N-diisopropylethylamine (DIEPA), diethylamine (NHEt2), guanidine derivatives, and bifunctional thiourea. A variety of inorganic bases including metal alkoxides, metal acetates, metal hydroxides, metal hydrides, metal carbonates, metal halides, and triphenylphosphine (TPP) can also be successfully used in Michael reactions.

Chemical route for synthesis of β-SiAlON:Eu2+ phosphors combining polymer-derived ceramics route with non-hydrolytic sol-gel chemistry

We report the synthesis of β-SiAlON:Eu2+ phosphors from novel single source precursors in which strictly controlled chemical composition is established at molecular scale. The two-step synthesis occurs by the chemical modification of perhydropolysilazane (PHPS) with Al(OCH(CH3)2)3 and AlCl3 in xylene at room temperature to 140 °C to introduce Al in the PHPS network while controlling the oxygen content followed by the reaction of EuCl2 with the Al-modified PHPS upon heat-treatment. Gas chromatography-mass spectrometry and thermogravimetry-mass spectrometry analyses revealed that PHPS reacted with Al(OCH(CH3)2)3 and AlCl3 via the formation of Al-N bonds. Moreover, AlCl3 reacted with nitrogen-bonded Al alkoxide residue to release 2-chloropropane in an analogy to the non-hydrolytic sol-gel reaction between metal alkoxide and metal chloride. Subsequently, AlCl3 acted as a Lewis acid catalyst to promote the Friedel-Craft alkylation between xylene solvent and the 2-chloropropane formed in-situ to afford dimethylcumene. On the other hand, EuCl2 reacted with silylamino moiety to form Eu-N bonds at around 850 °C. β-SiAlON:Eu2+ phosphors were successfully synthesized by pyrolysis of the precursors under flowing N2 or NH3 at 1000 °C, followed by heat treatment at 1800 °C for 1 h under a N2 gas pressure at 980 kPa. The polymer-derived β-SiAlON:Eu2+ (z = 0.55, Eu2+ 0.37 at%) exhibited green emission under excitation at 410 or 460 nm, and the green emission intensity under the excitation at 410 nm was found to be increased by reducing the carbon and chlorine impurities through the polymer-derived ceramics route investigated in this study.

Inherent selective pulsed chemical vapor deposition of amorphous hafnium oxide / titanium oxide nanolaminates

Water-free Inherent selective pulsed chemical vapor deposition (CVD) of HfO2/TiO2 nanolaminates on Si and SiO2 in preference to SiCOH has been studied. SiCOH is highly porous alkylated SiO2, which is used as a nonreactive low-k dielectric. Ti(OiPr)4 [titanium(IV) isopropoxide] and Hf(OtBu)4 [Hafnium tert butoxide] were used in the CVD study. Previous studies showed that metal alkoxide precursors could form oxide films through thermal decomposition. However, single oxide films greater than 2 nm can be rough due to crystallization. To solve this issue, HfO2/TiO2 nanolaminate structures were studied. With sequential dosing of each precursor in a supercycle at 300 °C sample temperature, HfO2/TiO2 nanolaminate films with thin (less than 3 nm) sublayers were selectively deposited. The films were smooth with root mean square (RMS) roughness lower than 0.5 nm and almost amorphous from XRD analysis; this is unexpected since both oxides readily crystalize. Amorphous nanolaminate oxide film deposition with high selectivity was achieved by controlling each sublayer thickness and the Hf:Ti ratio. TEM studies proved that ∼ 20 nm of the nanolaminate film could also be selectively deposited on nanoscale patterned surfaces. This selective amorphous nanolaminate oxide CVD process has a potential to be applicable in the nanoscale patterning in MOSFET fabrication.

Synthesis of highly active solid base catalysts of SrO-ZrO2 and SrO-SiO2 by solid–liquid interface reactions of hydrated strontium hydroxide with metal alkoxides

Highly dispersed SrO in amorphous ZrO2 or SiO2, SrO–ZrO2, and SrO–SiO2 mixed oxides were synthesized by applying the solid–liquid interface reaction of Sr(OH)2•8 H2O in the solid phase with an equal number of moles of Zr(OC3H7)4 or with twice the number of Si(OC2H5)4 moles dissolved in 2-propanol. After heat treating at 723 K, the SrO–ZrO2 catalyst exhibited a higher activity than the conventional solid base MgO catalyst for the base-catalyzed retroaldol reaction of diacetone alcohol. The catalytic activity of SrO–ZrO2 was higher by a factor of 1.5 than that of MgO prepared by the thermal decomposition of hydrated magnesium oxalate (MgC2O4•2H2O). By contrast, the highest activity of SrO–SiO2 obtained by thermal activation at 673 K was much lower than that of MgO. The active SrO–ZrO2 catalyst was composed of catalytically inactive species of SrZrO3, and small amounts of Sr(OH)2 and SrCO3. The expected active species of SrO was not detected in powder X-ray diffraction analysis in any sample. The formation of SrO in small crystal size dispersed in amorphous ZrO2 was strongly expected. Similar results were obtained in the SrO–SiO2 sample. An active site formed on SrO–ZrO2 surface is proposed.

Mechanistic Studies Yield Improved Protocols for Base-Catalyzed Anti-Markovnikov Alcohol Addition Reactions.

The catalytic anti-Markovnikov addition of alcohols to simple alkenes is a longstanding synthetic challenge. We recently disclosed the use of organic superbase catalysis for the nucleophilic addition of alcohols to activated styrene derivatives. This article describes mechanistic studies on this reversible reaction, including thermodynamic and kinetic profiling as well as computational modeling. Our findings show the negative entropy of addition is counterbalanced by an enthalpy that is most favored in nonpolar solvents. However, a large negative alcohol rate order under these conditions indicates excess alcohol sequesters the active alkoxide ion pairs, slowing the reaction rate. These observations led to an unexpected solution to a thermodynamically challenging reaction: use of less alcohol enables faster addition, which in turn allows for lower reaction temperatures to counteract Le Chatelier's principle. Thus, our original method has been improved with new protocols that do not require excess alcohol stoichiometry, enable an expanded alkene substrate scope, and allow for the use of more practical catalyst systems. The generality of this insight for other challenging hydroetherification reactions is also demonstrated through new alkenol cyclization and oxa-Michael addition reactions.

Synthetic Routes to Crystalline Complex Metal Alkyl Carbonates and Hydroxycarbonates via Sol–Gel Chemistry—Perspectives for Advanced Materials in Catalysis

Metal alkoxides are easily available and versatile precursors for functional materials, such as solid catalysts. However, the poor solubility of metal alkoxides in organic solvents usually hinders their facile application in sol–gel processes and complicates access to complex carbonate or oxidic compounds after hydrolysis of the precursors. In our contribution we have therefore shown three different solubilization strategies for metal alkoxides, namely the derivatization, the hetero-metallization and CO2 insertion. The latter strategy leads to a stoichiometric insertion of CO2 into the metal–oxygen bond of the alkoxide and the subsequent formation of metal alkyl carbonates. These precursors can then be employed advantageously in sol–gel chemistry and, after controlled hydrolysis, result in chemically defined crystalline carbonates and hydroxycarbonates. Cu- and Zn-containing carbonates and hydroxycarbonates were used in an exemplary study for the synthesis of Cu/Zn-based bulk catalysts for methanol synthesis with a final comparable catalytic activity to commercial standard reference catalysts.

Selective Recognition of Caffeine by (3-aminopropyl) Triethoxysilane Based Polymers

Background: The metal alkoxides undergo the sol-gel process leads to highly dense polymer networks in nano range size particles. Template-mediated nanomaterial fabrication involves functional monomers, cross binders, catalysis, and other favorable thermal conditions optimized to prepare thermal, mechanical, and chemo-stable tailor-made complimentary recognition sites ceramic materials. Methods: The present study is focused on imprinting of caffeine by sol-gel method, wherein functional monomer as (3-aminopropyl) triethoxysilane (APTES), crosslinker as tetraethyl orthosilicate (TEOS), and ammonium hydroxide solution as catalyst were considered to prepare materials, and those materials characterized. The catalyzed hydrolysis and condensation were carried with a wide range of composition mixtures in acetonitrile optimized at 60 oC. The imprinted polymers (MIPs) and Non-Imprinted Polymers (NIPs) characterized by FTIR, GC-MS, SEM, and EDS techniques. Further post-polymerization studies were carried to investigate recognizing capacity specificity and the binding capacity of the caffeine and its structurally similar analogs with imprinted / non-imprinted polymers. Results: The studies revealed that efficiency in the template removal was high in the imprinted polymers in which low concentration of crosslinker and whereas the selectivity of the template was observed to be high compared with polymers with higher concentrations. Conclusion: In conclusion, high binding affinity was observed in imprinted polymers to that of non- Imprinted polymers.

Nonaqueous Chemistry of Group 4 Oxo Clusters and Colloidal Metal Oxide Nanocrystals.

We review the nonaqueous precursor chemistry of the group 4 metals to gain insight into the formation of their oxo clusters and colloidal oxide nanocrystals. We first describe the properties and structures of titanium, zirconium, and hafnium oxides. Second, we introduce the different precursors that are used in the synthesis of oxo clusters and oxide nanocrystals. We review the structures of group 4 metal halides and alkoxides and their reactivity toward alcohols, carboxylic acids, etc. Third, we discuss fully condensed and atomically precise metal oxo clusters that could serve as nanocrystal models. By comparing the reaction conditions and reagents, we provide insight into the relationship between the cluster structure and the nature of the carboxylate capping ligands. We also briefly discuss the use of oxo clusters. Finally, we review the nonaqueous synthesis of group 4 oxide nanocrystals, including both surfactant-free and surfactant-assisted syntheses. We focus on their precursor chemistry and surface chemistry. By putting these results together, we connect the dots and obtain more insight into the fascinating chemistry of the group 4 metals. At the same time, we also identify gaps in our knowledge and thus areas for future research.

En route from metal alkoxides to metal oxides: metal oxo/alkoxo clusters

Molecular metal oxo or oxo/alkoxo clusters, MwOx(OH/OR)y(L/X)z (L or X = organic ligands), can often be isolated upon (partial) hydrolysis of metal alkoxides. Investigation of such clusters leads to a better understanding of the basic processes of sol–gel chemistry. The ligands not only stabilize the cluster core but also influence to some extent the cluster structures. They can easily change their position on the cluster surface, thus adapting to changing cluster geometries, and can be exchanged under certain conditions. A close inspection of titanium oxo/alkoxo cluster structures, taken as an informative example, shows that Ti3O units (with or without organic ligands) are the basic building blocks. Clusters with higher nuclearities appear to be predominantly formed by cluster–cluster or by cluster–monomer condensations. Ligand substitution or condensation reactions within a cluster unit are also possible.Instead of extended metal oxide networks, metal oxo/alkoxo clusters may be formed upon hydrolysis of metal alkoxides.

Hard-templated engineering of versatile 2D amorphous metal oxide nanosheets

Two-dimensional (2D) nanomaterials have received ever-increasing attention and in-depth exploration in multifarious fields on account of their superior mass transfer ability and abundant catalytic-active sites. Especially, the amorphous 2D nanomaterials feature unique properties distinct from atomic crystalline materials. However, the synthesis of high-quality and large-sized amorphous 2D nanomaterials encounters a big challenge. Here, a general and facile synthetic strategy for a series of 2D amorphous metal and nonmetallic oxides nanosheets, including SiO2, AlOOH, ZrO2 and TiO2 nanosheets, is reported. The versatile 2D amorphous nanomaterials are fabricated via manipulating the surface energy of relevant metal alkoxide precursors with liquid feature and controlling the related synthesis parameters to form solid 2D amorphous nanosheets by in situ hydrolysis and condensation of precursors. Density functional theory (DFT) calculations reveal the molecular adsorption mechanism of wetting process of precursor infiltrated on solid NaCl substrate, which attributes to the strong interaction between Na–O atom pairs from NaCl and metal alkoxides respectively. Furthermore, taking the 2D Fe–ZrO2 nanomaterials as the catalyst, the excellent catalytic performance for Rhodamine B (RhB) degradation illustrates that these 2D nanomaterials prepared by this method have the characteristics of easy functionalization. This work provides an efficient strategy for nanomaterials functionalization during 2D nanosheets synthetic process and further being applied in catalysis-related field and beyond.

Synthesis of Mg Alkoxide Nanowires from Mg Alkoxide Nanoparticles upon Ligand Exchange.

We report on a new synthesis pathway for Mg n-propoxide nanowires (NWs) from Mg ethoxide nanoparticles using a simple alkoxy ligand exchange reaction followed by condensation polymerization in n-propanol. In order to uncover the morphology-structure correlation in the metal alkoxide family, we employed a powerful range of state-of-the-art characterization techniques. The morphology transformation from nanoparticles to nanowires was demonstrated by time-lapse SEM micrographs. Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (such as 1H NMR and solid-state 13C cross-polarization (CP)-MAS NMR) illustrated the replacement of ethyl by n-propyl and metal alkoxide condensation polymerization. We identified chemical formulas of the products also using NMR spectroscopy. The crystal structure simulation of Mg ethoxide particles and Mg n-propoxide NWs provided insights on how the ligand exchange and the associated increase in the fraction of OH groups greatly enhanced Mg alkoxide bonding and enabled a higher degree of coordination polymerization to facilitate the formation and growth of the Mg n-propoxide NWs. The discovered synthesis method could be extended for the fabrication of other metal alkoxide (nano) structures with various morphologies.

Activation of polar organometallic reagents with alkali-metal alkoxides

The use of group 1 metal alkoxides as additives to activate s-block organometallics is an established phenomenon in polar organometallic chemistry. Typified by the Lochmann–Schlosser superbase, these reagents have proved to be exceptionally powerful bases for the deprotonative metallation of organic substrates. However, despite their long-standing importance in synthesis, the nature of this activating effect remains to be fully understood. Here we shed light on the origins of the special reactivities of which these reagents are capable, which generally cannot be replicated by their homometallic precursors. In addition, reactivity studies that have established the mixed-metal constitutions of these organometallic reagents are discussed. Opening up new directions in synthesis, the use of lithium alkoxides as additives to promote direct regioselective magnesium (or zinc)/halogen exchange processes to access highly functionalized organometallics is also discussed, with an emphasis on rationalizing the role of each metal in these transformations.

Alkali-free synthesis of hexagonal star-like Fe-ethylene glycol (Fe-EG) complex and subsequently decomposition to α-Fe2O3 and Fe3O4/α-Fe/C composites

• Hexagonal star-like Fe-EG complex was synthesized by a surfactant-and alkali-free solvothermal method. • Hexagonal star-like α-Fe 2 O 3 and Fe 3 O 4 /α-Fe/C were obtained after thermal treatment. • The saturation magnetisation of α-Fe 2 O 3 and Fe 3 O 4 /α-Fe/C were 17.07 and 86.95 emu g −1 , respectively. • The adsorption capacity of Fe 3 O 4 /α-Fe/C towards Congo red was 230.9 mg g −1 . Using ferric nitrate as iron source, hexagonal star-like Fe-ethylene glycol (Fe-EG) complex was synthesized using an alkali-and surfactant-free solvothermal method. The formation of the Fe-EG complex was attributed to the hydrolysis of ferric ions, the oxidation-reduction between EG molecules and nitric acid derived from the hydrolysis of ferric ions, and the reaction between the Fe-O sheets and the deprotonated hydroxyl and carboxyl groups. Hexagonal star-like α-Fe 2 O 3 and Fe 3 O 4 /α-Fe/C composites were obtained after thermal treatment. The saturation magnetisation of them were 17.07 and 86.95 emu g −1 , respectively. The adsorption of the obtained Fe 3 O 4 /α-Fe/C composites towards Congo red (CR) can be well fitted by the Langmuir model, and the maximum adsorption capacity was 230.9 mg g −1 . The results put forward an effective approach for the synthesis of metal alkoxides with a special microstructures.

Rheological Properties of MWCNT-Doped Titanium-Oxo-Alkoxide Gel Materials for Fiber Drawing.

A strategy of doping by multi-walled carbon nanotubes (MWCNT) to enhance mechanical strength and the electrical conductivity of ceramic fibers has nowadays attracted a great deal of attention for a wide variety of industrial applications. This study focuses on the effect of MWCNTs on rheological properties of metal alkoxide precursors used for the preparation of nanoceramic metal oxide fibers. The rheological behavior of MWCNT-loaded titanium alkoxide sol precursors has been evaluated via an extensional rheometry method. A substantial decrease in elongational viscosity and relaxation time has been observed upon an introduction of MWCNTs even of low concentrations (less than 0.1 wt.%). A high quality MWCNT/nanoceramic TiO2 composite fibers drawn from the specified precursors has been validated. The MWCNT percolation, which is mandatory for electrical conductivity (50 S/m), has been achieved at 1 wt.% MWCNT doping.

Anionic ring‐opening polymerization behavior of trans‐cyclohexene carbonate using metal tert‐butoxides: Construction of living anionic ring‐opening polymerization by lithium tert‐butoxide

AbstractAnionic ring‐opening polymerization (ROP) behavior of trans‐cyclohexene carbonate (CHC) using metal alkoxides as initiators was investigated. As a result, lithium tert‐butoxide‐initiated ROP of CHC with a high‐monomer concentration (10 M) at low temperature (−15 to −10°C) proceeded to afford a poly(trans‐cyclohexene carbonate) (PCHC) without undesired side reactions such as mainly backbiting. The suppression of side reactions enables the control of the molecular weight (Mn = 2400–6100) of PCHC with low molar‐mass dispersity values (Mw/Mn = 1.16–1.22). Furthermore, by increasing the feed ratio of the monomer to the initiator, the molecular weight increases proportionally, indicating a controllable polymerization. The results of a matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry analysis, a kinetic study, and a chain extension experiment suggested a living nature of this ROP using lithium tert‐butoxide.

Stabilized and Almost Dendrite-Free Li Metal Anodes by In Situ Construction of a Composite Protective Layer for Li Metal Batteries.

Li metal anodes (LMAs) are promising candidates for the anodes of high-energy-density batteries due to their lower reduction potential and high specific capacity. Unfortunately, LMAs usually suffer from uncontrollable Li plating and insecure solid electrolyte interphase layers, especially when used in conjunction with carbonate-based electrolytes. Herein, we proposed using metal alkoxides of titanium butyrate to react with hydroxyl groups on Li metal. A composite protective layer containing TiO2 and ROLi was generated to modify Li (designated as treated Li), leading to dendrite-free LMAs and achieving significantly enhanced cycling stability. Notably, symmetric cells using treated Li electrodes can deliver over 1500 h of stable cycling under a current density of 2 mA cm-2 in an ether-based electrolyte. Moreover, under extreme conditions of 5 mA cm-2 using a carbonate-based electrolyte, symmetric cells employing a treated Li electrode demonstrated stable cycling for over 80 h, as compared to the fluctuating voltage seen after only 10 h of cycling when using a bare Li electrode. Furthermore, full cells using a treated Li anode coupled with a high loading of LiCoO2 cathode (≈15 mg cm-2) displayed excellent cycling stability at 0.2 C over 150 cycles with a high capacity retention of 98.1% and an enhanced average Coulombic efficiency above 99.6%. By comparison, full cells using the bare Li anode drop to 125.4 mA h g-1 with a capacity retention of just 83.3%. The treated Li exhibited superior rate performance and delivered 132.7 mA h g-1 even at 5 C. This strategy provided a facile and effective option for the construction of advanced LMAs.

Synthesis of organic carbamates as polyurethane raw materials from CO2: the quest for metal alkoxides as regenerable reagents.

It is well known that the utilization of carbon dioxide (CO2) for chemical materials is attracting research attention from the viewpoint of the carbon cycle. To contribute to the reduction of CO2 emission through such CO2 utilization reactions and counteract global climate change, the target compounds should be core chemical products that are distributed in large quantities and used for a long time. One such synthetic target is isocyanates that are used as raw materials for the production of polyurethanes, which are versatile polymeric materials with a service life of approximately 10 years. However, since direct synthesis of isocyanate from CO2 is quite difficult due to equilibrium constraints, a method via the use of its alcohol adduct, organic carbamate, as a precursor has been proposed. In this Perspective, we present regenerative metal alkoxide reactants, such as tin alkoxide, titanium alkoxide, and alkoxysilane, as environmentally benign reactants for the synthesis of organic carbamates from CO2. We also present a practical and environmentally friendly method for the highly efficient synthesis of various organic carbamates, including industrially important diisocyanate precursors, from 1 atm CO2 using alkoxysilanes. Furthermore, prospects for the practical application of these carbamate synthesis reactions are also discussed.

Iodine and alkali metal alkoxides: a simple and versatile catalytic system for fully alternating polyester synthesis from phthalic anhydride and epoxides

Regioselective ROCOP of various epoxides with phthalic anhydride using readily available and economical catalysts i.e. I2 in combination with alkali metal alkoxides was used.